Conventional carbon fiber prepregs comprising carbon fiber bundles composed of numerous carbon filaments and a matrix resin were, even when their constituent carbon fiber bundles had been drawn together in one direction, plagued with the presence of localized aggregation of the carbon filaments within the carbon fiber bundles, localized positional fluctuation of the carbon fiber bundles, localized twisting of the carbon filaments within the carbon fiber bundles, and the like on their surfaces. As a result, moldings (carbon fiber reinforced composite materials) molded from them had poor design performance from the viewpoint of uniformity in the external appearance of their surfaces.
Unevenness of dispersion of the carbon filaments resulting from a localized aggregation of the carbon filaments within the carbon fiber bundles generate a fluctuation in color shade on a surface of a molding. Positional fluctuation, twisting and other forms of fluctuation in the orientation of the carbon filaments, on the other hand, cause an unpleasant feeling to people looking at such a surface of the molding because it changes its appearance according to the incidence angle of light falling on the surface. Often described with such words as “visual irritation”, “glistening” and “fluctuation”, such an unpleasant feeling annoys people, giving rise to calls for moldings free of such defects.
Since this unpleasant feeling stems from visible unevenness on the surfaces of moldings, attempts have been made to disguise them through the application of a coat to their surfaces and other techniques. However, such an additional surface treatment is problematic in that it increases the weight of moldings, as well as failing to make the most of the design advantage of carbon fiber. The application of a transparent coat (clear coating), which leaves the surface of the molding fully visible, on the other hand, had no effect on the above unpleasant feeling, and this has led to calls to eliminate it through an improvement in design performance.
As can be seen from patent documents 1 to 11, there are conventional techniques to obscure unevenness, such as controlled resin flow based on the restriction of the flow of the resin during the molding process and covering of the mold surface with a translucent glass scrim cloth. However, these are all based on an improvement in molding performance in terms of managing the generation of defects that greatly impair external appearance. Namely, rather than improving the design performance of carbon fiber reinforced composite materials, they merely alleviate molding defects.